This invention relates to a magnetic core, and more particularly, to a magnetic core which is excellent in the frequency characteristic of magnetic permeability and also has a high magnetic flux density. It also relates to a method of producing the magnetic core.
In the prior art, in electrical instruments such as an electric power converting device, including a device for converting an alternating current to a direct current, a device for converting an alternating current having a certain frequency to another alternating current having a different frequency and a device for converting a direct current to an alternating current such as so called inverter, or a non-contact breaker, etc., there have been employed, as electrical circuit constituent elements thereof, semiconductor switching elements, typically thyristors and transistors, and reactors for relaxation of turn-on stress in a semiconductor switching element, reactors for forced commutation, reactors for energy accumulation or transformers for matching connected to these elements.
As an example of such electric power converting devices, FIG. 1 shows an electrical circuit of a device for converting a direct current to an alternating current. The electric power converting device shown in FIG. 1 includes a thyristor 1, a reactor for relaxation of turn-on stress of semiconductor switching element 2 and a transformer for matching 3. Numeral 4 designates load on alternating current and numeral 5 a direct current power source.
Through these reactors or transformers, a current containing a high frequency component reaching 100 KHz or higher, even to over 500 KHz in some cases, may sometimes pass on switching of the semiconductors.
As the magnetic core constituting such a reactor or a transformer, there have been employed in the prior art such materials as shown below. That is, there may be mentioned:
(a) a laminated magnetic core produced by laminating thin electromagnetic steel plates or permalloy plates having applied interlayer insulations;
(b) a so-called dust core produced by caking carbonyl iron minute powder or permalloy minute powder with the use of, for example, a resin such as a phenolic resin; or
(c) a so-called ferrite core produced by sintering an oxide type magnetic material.
Among these, a laminated magnetic core, while it exhibits excellent electric characteristics at a commercial frequency band, is marked in its iron loss at higher frequency band, particularly increased eddy-current loss, in proportion to the square of a frequency. Another property is that the magnetizing power can resist change at inner portions farther from the surface of plate materials constituting the magnetic core because of the eddy-current of the magnetic core material. Accordingly, a laminated magnetic core can be used only at a magnetic flux density which is far lower than the saturated magnetic flux density inherently possessed by the magnetic core material itself, and there is also involved the problem of very great eddy-current loss. Further, a laminated magnetic core has a problem of extremely lower effective magnetic permeability relative to higher frequency, as compared with that relative to commercial frequency. When a laminated magnetic core having these problems is used in a reactor, a transformer, etc. connected to a semiconductor switching element through which a current having a high frequency component passes, the magnetic core itself must be of large dimensions to compensate for effective magnetic permeability and magnetic flux density, whereby, also because of lower effective magnetic permeability, there is also involved the problem of increased copper loss.
On the other hand, there is employed as a magnetic core material, a compressed powdery magnetic body called a dust core, as described in detail in, for example, Japanese Pat. No. 112235. However, such dust cores generally have considerably lower values of magnetic flux and magnetic permeability. Among them, even a dust core using carbonyl iron powder having a relatively higher magnetic flux density has a magnetic flux density of only about 0.1 T and a magnetic permeability of only about 1.25.times.10.sup.-5 H/m at a magnetizing force of 10000 A/m. Accordingly, in a reactor or a transformer using a dust core as the magnetic core material, the magnetic core must inevitably be of large dimensions, whereby there is involved the problem of increased copper loss in a reactor or a transformer.
Alternatively, a ferrite core employed in a small scale electrical instrument has a high resistivity value and a relatively excellent high frequency characteristic. However, a ferrite core has a magnetic flux density as low as about 0.4 T at a magnetizing force of 10000 A/m, and the values of magnetic permeability and the magnetic flux density at the same magnetizing force are respectively varied by some ten percent at -40.degree. to 120.degree. C., which is the temperature range useful for the magnetic core. For this reason, when a ferrite core is to be used as an magnetic core material for a reactor or a transformer connected to a semiconductor switching element, the magnetic core must be enlarged because of the small magnetic flux density. But, a ferrite core, which is a sintered product, can be produced with a great size only with difficulty and thus is not suitable as the magnetic core. Also, a ferrite core involves the problems of great copper loss caused by its low magnetic flux density, of its great characteristic change when applied for a reactor or a transformer due to the great influence by temperatures on magnetic permeability and magnetic flux density, and further of increased noise generated from the magnetic core due to the greater magnetic distortion, as compared with an silicon steal, etc.
An object of this invention is to provide a magnetic core to be used for a reactor or a transformer connected to a semiconductor element, which has overcome the problems described above, and also has both an excellent frequency characteristic of magnetic permeability and a high magnetic flux density.